Hello All.
I'd like to start by stating I'm not familair with most of terms with this type of thing but I'm versed in Mechanical Engineering so if you speak in layman's terms I'm sure I'll be just fine.

Now. If I bring the SPA, the phone cable and the phone to my workplace....enter the IP info into the SPA....the phone works fine. I make calls with 2-way audio and can receive calls 2-way audio.

I bring it home and edit the IP in SPA I only get 1-way audio. Incoming OR outgoing calls I hear nothing where the other end hears everything. I tried port range forwarding 16385-16482 (Both) and enter the IP of the SPA....no change. I've changed both Ethernet cables with new ones. From the modem to router and router to SPA. No change.

I'll leave the post here. Let me know if you need more info and I'll give it a try.

Are you able to do a test with the SPA plugged directly into the Thomson modem rather than through the Linsksys? If that gives you two way audio at least you'll know that it's something on the Linksys that needs to be tweaked.

I have had a problem in the past with a SIP Application Layer Gateway on a Linksys router. That's not to say that's the issue here but it is one of a few things it could be. First step is to determine if audio works without the Linksys router.

Having the SPA plugged directly to the Thomson 475.....I get no dial tone and subsequently cannot dial out.
Plugging the router back to the modem and the SPA back into the router gets me the dial tone back but still one way audio.

One-Way Audio:
When I speak into my cell phone my voice echos in the cell phone.
During that same call if I speak into the land line headset I hear my voice in my cell phone only.
There is never audio on the land line headset.

Me again
Recently the phone line no longer allows calls. Once again I can call into the land line from a cell phone, the land line rings and I can hear voice on the land line but speaking into the land line does not come through on the cell line.
When I call out from the land line I get a dial tone, the phone number dials and then nothing else happens. Dead air and never connects.
Any thoughts on what to check?

I know it's a big hassle but I'd be tempted to try a different router (or even better just a plain switch) in place of your Linksys device.

If it's messing around with your VoIP traffic, whether it be SIP or RTP, it's going to be a headache to sort out. It's usually better to keep your network devices as simple as possible and I suspect your Thompson modem could do your DHCP (allocate your local IP addresses for you) and firewalling in which case you can dispense with the Linksys and replace it with a $30 (or less) switch which won't know anything about SIP and RTP and therefore won't mess around with it.